Conventionally, various linear light sources are used in image reading apparatus. FIG. 7A illustrates an example of prior-art linear light source. The light source includes an insulating rectangular substrate 90 and a plurality of LEDs (light emitting diodes) 92. The substrate 90 has a surface formed with a wiring pattern 8 (darkened portion). The LEDs 92 are arranged at a predetermined pitch in a row extending longitudinally of the substrate 90.
The wiring pattern 8 includes two wiring portions extending longitudinally of the substrate 92, i.e., a first wiring portion 80 and a second wiring portion 81. The first wiring portion 80 has one end connected to a first terminal 83a, whereas the second wiring portion 81 has one end connected to a second terminal 83b. The above-described row of LEDs 92 is arranged between the first wiring portion 80 and the second wiring portion 81. The wiring pattern 8 further includes a plurality of auxiliary wiring portions 82 which are also arranged between the first wiring portion 80 and the second wiring portion 81.
As shown in FIG. 7A, each auxiliary wiring portion 82 has a left end at which is disposed one LED 92, which is connected to the first wiring portion 80 via a wire W. Other LEDs 92 are disposed at predetermined portions of the second wiring portion 81. Each of these LEDs 92 is connected via a wire W to the right end of the corresponding auxiliary wiring portion 82.
FIG. 7B is a circuit diagram of the above-described linear light source. As will be understood from this figure, the plurality of LEDs 92 are divided into a plurality of groups b1˜bn (each group including two LEDs 92 connected in series). These groups b1˜bn are connected in parallel to each other with respect to the first and the second wiring portions 80, 81. With this structure, when a predetermined voltage is applied across the first and the second terminals 83a, 83b, the plurality of LEDs 92 emit light.
Although the above-described prior art linear light source can be used in various types of image reading apparatus, it has the following problems. Although the wiring pattern 8 conducts a current well, the electrical resistance is not zero. Therefore, in the prior art structure shown in FIG. 7A, due to this inherent electrical resistance, the luminosity of light emitted from the LEDs 92 which belong to one group (e.g., group b1) does not equal to the luminosity of light emitted from LEDs 92 which belong to another group (e.g., group b3).
In more detail with reference to FIG. 7B, since the wiring pattern 8 itself has an electrical resistance, the first wiring portion 80 has a resistance RV between adjacent groups (bi and bi+1), whereas the second wiring portion 81 has a resistance RG between adjacent groups (bi and bi+1). Therefore, a voltage applied to the LEDs in a group located farther from the first and the second terminals 83a, 83b becomes smaller than the voltage applied to the LEDs in a group located closer to the terminals. As a result, LEDs in each group emit a different amount of light from each other. Specifically, the LEDs 92 which belong to the group b1 emit the brightest light whereas the LEDs 92 in the group bn emits the weakest light. Light emitted from the LEDs becomes gradually weaker from the group b2 to the group bn−1.
With the prior art linear light source having the above-described characteristics, it is impossible to uniformly illuminate a document to be read with light, so that an appropriately read image may not be obtained. Such a problem occurs notably when an image reading apparatus is driven at a low voltage (for example, 5V).